Why do “Brood X” cicadas have such a strange life cycle?

NEW YORKERS, having spent months sheltering from covid-19, are emerging again. From May 17th the subway will resume running 24-hours a day and two days later restaurants can reopen at full capacity. But humans are not the only creatures surging onto the streets after a long time hidden away. A combination of suitable soil temperature and weather has prompted billions of cicadas, known as Brood X, to begin swarming large parts of the eastern United States. The insects last emerged in 2004, and although their loud mating calls can be distracting, they are not directly harmful to people. Why do the cicadas have such an unusual life cycle?

Brood X emerges around this time of year every 17 years, and is the largest of a number of broods of so-called periodical cicadas. Others emerge at intervals of 13 years. During that time cicada nymphs remain underground, feeding on sap from tree roots until their biological alarm clocks go off. When that happens they mature, emerge, mate, lay eggs if female and then die—all in four to six weeks. Climate change may be causing them to emerge earlier in the year, however, as spring temperatures rise. Gene Kritsky, a biologist at Mount St Joseph University in Cincinnati, has observed that the typical arrival of Brood X has moved from late May in the first half of the 20th century to the first two weeks of the month today.

Biologists believe that the strange lifestyle of periodical cicadas is an example of a survival strategy called “predator satiation”. The insects emerge in such prodigious quantities that predators, such as birds and rodents, cannot possibly eat them all. Their prime-numbered life cycles may also help cicadas avoid damaging “resonances” with the two- and three-year population fluctuations of their predators. An abundance of prey allows predators to reproduce in greater numbers, so for example birds with a life cycle that is a factor of the cicadas’ would be in abundance the next time they emerged. This could establish a rhythm resulting in lots of predators regularly being around in years when there were lots of prey.

Female cicadas must pick a healthy tree in which to lay their eggs (to the vexation of orchard owners, whose trees can be weakened by the cicadas). They deposit them in the tree’s branches. When they hatch two weeks later, the larvae fall to the ground. Next they tunnel down to the tree’s roots, from which they suck out their sustenance. If a female chooses poorly and the tree dies in the next 17 years, her larvae will perish too. They will also do badly if the tree has old, gnarled roots, rather than young, succulent ones. This may explain why cicadas do well in suburban areas, where trees tend to be young, healthy and face less competition for resources than they might in a thicket.

Learning about the members of Brood X is a painstaking process, not least because they only emerge every 17 years. Entomologists have been gathering data on this group for over a century, but modern technology will give them a helping hand this time. Mr Kritsky has developed an app called Cicada Safari through which the public can upload cicada sightings, helping to map the creatures’ emergence and habitats. This year, the sound of smartphone-cameras snapping may be audible alongside the cicadas’ calls.